Provider: add warm up method

So that the zygote can warm up the providers, avoiding each app
to do the warm up in the first call to get a service.

(cherry picked from commit e9cdd78f586994cb3a7d3545e2312afbec7b2a8f)

Bug: 28545496
Change-Id: Ic5aab3ece609d9fef06fee4ccb83d8371af075b2
1 file changed